Make room alias lists peekable

As per
https://github.com/matrix-org/matrix-doc/pull/2432#pullrequestreview-360566830,
make room alias lists accessible to users outside world_readable rooms.

synapse/handlers/directory.py

@@ -463,7 +463,9 @@ class DirectoryHandler(BaseHandler):
         # allow access to server admins and current members of the room
         is_admin = await self.auth.is_server_admin(requester.user)
         if not is_admin:
-            await self.auth.check_joined_room(room_id, requester.user.to_string())
+            await self.auth.check_user_in_room_or_world_readable(
+                room_id, requester.user.to_string()
+            )
 
         aliases = await self.store.get_aliases_for_room(room_id)
         return aliases

tests/rest/client/v1/test_rooms.py

@@ -1766,6 +1766,23 @@ class DirectoryTestCase(unittest.HomeserverTestCase):
         res = self._get_aliases(self.room_owner_tok)
         self.assertEqual(set(res["aliases"]), {alias1, alias2})
 
+    def test_peekable_room(self):
+        alias1 = self._random_alias()
+        self._set_alias_via_directory(alias1)
+
+        self.helper.send_state(
+            self.room_id,
+            EventTypes.RoomHistoryVisibility,
+            body={"history_visibility": "world_readable"},
+            tok=self.room_owner_tok,
+        )
+
+        self.register_user("user", "test")
+        user_tok = self.login("user", "test")
+
+        res = self._get_aliases(user_tok)
+        self.assertEqual(res["aliases"], [alias1])
+
     def _get_aliases(self, access_token: str, expected_code: int = 200) -> JsonDict:
         """Calls the endpoint under test. returns the json response object."""
         request, channel = self.make_request(